1. Field of the Invention
The present invention relates to co-vulcanizable compositions of fluoroelastomers.
More particularly, the present invention relates to co-vulcanizable compositions of fluoroelastomers, having improved chemical stability.
2. Description of the Prior Art
The fluoroelastomers on the basis of vinylidene fluoride, vulcanized according to the bisphenolic or dihydroxy systems (Kirk-Othmer, Encyclopaedia of Chemical Technology, vol. 8, page 506, 1979), are very valuable products which are used in the industrial world, in particular in the fields of car and oil industry, and in general in the energy industrial field.
In the automobile field, fluoroelastomers are used mainly in the fuel circulation system, or as components of engine parts and of parts of gearbox-differential-transmission unit.
In the first case, as application examples, membranes for fuel pumps, valve seats, non-return valves, flexible hoses and, in general, seal gaskets for fuel can be mentioned.
In the second case, analogous examples are gaskets for gear wheels, seal liners for cylinders in diesel engines, gaskets for the crankshaft, non-return valves for the exhaust gases, gaskets of the type of shaft seals, and still others.
In the oil and energy industrial field, fluoroelastomers are used for applications wherein they come into direct contact with the crude oil, above all at high temperatures and pressures, such as, for example, for gaskets, valves and pumps, or parts thereof.
In this field, an important application for fluoroelastomers is represented by the expansion joints used in the exhaust ducts for the flue gases in thermoelectric power plants, wherein resistance to high temperatures associated with resistance to the corrosive action of acidic flue gases is required.
Fluoroelastomers are used in these, and in other fields, such as in the electric and electronic field, thanks to their properties of high thermal stability and chemical resistance, even at high temperatures.
But during the last years, and in some application types, precise technological requirements have arisen, which the manufactures of the prior art, vulcanized by the bisphenolic or dihydroxy systems, are not able to fulfil.
The problematic parameter is in particular the chemical stability under severe operating temperature conditions in the presence of steam, of mineral acids, of highly aggressive lubricating fluids, such as, e.g., the oils of SF series, containing as additives compounds having a high swelling capacity for fluoroelastomers based on vinylidene fluoride.
Furthermore, a general improvement is required as regards the processability characteristics of the mixes, for the moulding of such articles as the shaft seals, as well as the possibility to easily extrude the mixes to produce suitable semifinished articles having even complex profiles.
The problem of the chemical stability has been solved thanks to the use of the vulcanization process by peroxides.
This vulcanization type does not however allow the above mentioned requirements as for processability, in particular by moulding and extrusion, to be fulfilled; moreover, the same processability results less reliable (Rubber Chemistry and Technology, vol. 55, pages 906, 1982), and on compression set test much poorer results are obtained (Kirk-Othmer, ibid., page 510).
In European patent appln. No. 84.110707, co-vulcanized compositions have been prepared and disclosed, which have allowed certain improvements in performance to be achieved, but which still show poor processing reliability and slight decrease of chemical-thermal properties.